Not Applicable.
Not Applicable.
This invention relates to anti-friction bearings, and, in particular, to a coating for the bearing which will increase the bearings adhesive and abrasive wear resistance, and reduce the bearings operational temperature to enable the bearing to have a longer useful life.
In attempting to increase the efficiency, performance, and reliability of machinery, anti-friction bearings have approached the limits of their load bearing capacity. Additionally, bearings do not always operate in their ideal state of elasto-hydrodynamic lubrication, but in operating states that entail increased wear: such as mixed layer lubrication, boundary layer lubrication, dry operation, or wear caused by abrasive particles.
In elasto-hydrodynamic lubrication, the bearing""s rolling elements are separated by a supporting lubricating film which allows bearings to attain their maximum service life. Under normal operating conditions, the maximum service life is determined primarily by the strength of the bearing material.
In mixed or boundary layer lubrication situations, supporting lubricating films do not develop during operation. Under rolling and sliding contact, the asperities of the bearing elements (i.e., the rolling elements and the raceways) contact each other, the surfaces suffer from wear, and the service life of the bearing is reduced.
Although dry running is not desirable operating condition, rolling element bearings many times are subjected to accidental or momentary oil loss. Sometimes the excessive wear associated with this oil loss can potentially cause catastrophic damage to the machinery.
Today, the wear problems are primarily addressed with very pure grades of steel, sophisticated heat treatment methods, new lubricants, and improved seals. These wear problems can also be addressed by applying coatings with specific mechanical properties to the bearing elements.
I have found that when the rolling elements and raceways of anti-friction bearings are coated with an amorphous boron carbide coating, the operational performance of the bearings is increased dramatically. Boron carbide exhibits high wear resistance, dimensional accuracy, and toughness. Boron carbide with an approximate stoichiometry of B4C is the third hardest material currently known, behind diamond and cubic boron nitride. The extreme hardness of boron carbide makes it resistant to abrasion from debris, and wear resistant from the point of view of asperity contact. Boron carbide is most useful for mechanical components when it does not possess long range structural order. This incoherent, or amorphous form of boron carbide does not typically suffer from the large compressive stresses that other hard coatings possess. Consequently, tribological coatings with sufficient thickness and with excellent adhesive strengths to steel can be realized. Additionally, amorphous boron carbide coatings also have very good fracture toughness, and are resistant to mechanisms of crack propagation that occur in crystalline coatings.
The boron carbide coating can be applied through physical and/or chemical vapor deposition processes. Whereas coatings deposited by chemical vapor deposition processes are usually grown with very high substrate temperatures, boron carbide coatings deposited by physical vapor deposition processes can be grown at temperatures well below the tempering points of low alloy steels.
Boron carbide coatings can be applied to substrates with or without an adhesion enhancing or interfacial interlayer which is about 0.1 mm thick. The interfacial layer is a metal such as titanium or chromium. The interfacial layer may also be formed by diffusing boron and carbon atoms into the steel surface to be coated, for example, by applying a high bias voltage during the initial stages of deposition.
The coating itself is preferably about 0.1 xcexcm-5 xcexcm thick. It can be doped with semiconducting (e.g., Si) or metallic elements (e.g., Al, Ti, W, or Cr) which can be used to modify the mechanical properties of the coating should that prove to be desirable. Further modification of the mechanical properties of boron carbide can be accomplished by doping the coatings with gaseous elements (e.g., H, N).